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Boil some water on the stove top (only ~2 cups needed).
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Place the red onion slices in a medium-sized, glass or ceramic bowl. Pour the boiling water over the onions, stir, and let sit for 10 seconds. Drain the water from the onions.
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Add the lime juice, salt, oregano, and red pepper flakes into the onion bowl. Stir to combine.
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Transfer to an airtight container and store in the fridge. Let sit for at least 15 minutes before using. Enjoy!